Note 13 - Pension Plans and Other Postretirement Benefits - Amounts Recognized in the Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022
Noncurrent liabilities	$ (137.3)	$ (163.0)
Spire Missouri [Member]
Noncurrent liabilities	(103.3)	(115.5)
Spire Alabama Inc [Member]
Noncurrent liabilities	(27.2)	(40.5)
Pension Plan [Member]
Current liabilities	(1.6)	(1.1)
Noncurrent liabilities	(102.4)	(130.6)
Total	(104.0)	(131.7)
Pension Plan [Member] | Spire Missouri [Member]
Current liabilities	(1.6)	(1.1)
Noncurrent liabilities	(69.6)	(83.6)
Total	(71.2)	(84.7)
Pension Plan [Member] | Spire Alabama Inc [Member]
Current liabilities	0.0	0.0
Noncurrent liabilities	(27.0)	(40.5)
Total	(27.0)	(40.5)
Other Postretirement Benefits Plan [Member]
Current liabilities	(0.4)	(0.3)
Noncurrent liabilities	(36.6)	(34.7)
Noncurrent assets	180.7	151.6
Total	143.7	116.6
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member]
Current liabilities	(0.4)	(0.3)
Noncurrent liabilities	(36.6)	(34.7)
Noncurrent assets	120.5	96.9
Total	83.5	61.9
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member]
Current liabilities	0.0	0.0
Noncurrent liabilities	0.0	0.0
Noncurrent assets	56.9	53.4
Total	$ 56.9	$ 53.4